The existing jsoncomm is a work of beautiy. For very big arguments
however the used `SOCK_SEQPACKET` hits the limitations of the
kernel network buffer size (see also [0]). This lead to various
workarounds in #824,#1331,#1836 where parts of the request are
encoded as part of the json method call and parts are done via
a side-channel via fd-passing.
This commit changes the code so that the fd channel is automatically
and transparently created and the workarounds are removed. A test
is added that ensures that very big messages can be passed.
[0] https://github.com/osbuild/osbuild/pull/1833
0abdfb9041